Certain types of fiber optic systems require couplers in which at least a portion of the light propagating in an optical fiber is coupled to one or more output fibers. The present invention relates to such fiber optic couplers and more particularly to a cost effective and reproducible method of making such fiber optic couplers.
It has been known that coupling occurs between two closely spaced cores in a multiple core device. The coupling efficiency increases with decreasing core separation and, in the case of single-mode cores, with decreasing core diameter. A number of couplers that are based on these principles have been developed. Such couplers are capable of low loss operation; they typically exhibit an excess loss of about 1 dB or less.
Multimode and single-mode couplers have been formed by positioning a plurality of fibers in a side-by-side relationship along a suitable length thereof and fusing the claddings together to secure the fibers and reduce the spacings between the cores. Coupling can be enhanced by stretching and by rotating the fibers along the fused length thereof as taught in U.S. Pat. No. 4,426,215 to Murphy; however, rotating the fibers is disadvantageous for certain purposes. Also, a portion of the cladding is sometimes removed by etching or grinding to decrease the intercore distance as taught in U.S. Pat. No. 4,449,781 to Lightstone et al. Since the coupling region is fragile and is exposed to the atmosphere, such couplers must then be provided with a hermetic enclosure. These processes are labor intensive and therefore expensive, they may lack long term integrety, and do not always result in couplers exhibiting predetermined desired coupling characteristics. Such disadvantages are particularly apparent in the manufacture of certain single-mode couplers wherein the coupling core sections are to remain parallel to each other to insure that the propagation constants are matched and of certain single-mode couplers which must possess optical characteristics such as polarization retention.
Although most couplers are made by applying heat directly to the fibers to be joined, U.S. Pat. No. 3,579,316 to Dyott et al. teaches a method wherein the fibers are first inserted into a capillary tube where the ends may overlap. The capillary tube is formed of a glass having a refractive index lower than that of the fiber cladding material. Heat is applied to the capillary tube in the vicinity of the fiber overlap and the tube is stretched until the diameter thereof approximates that of the original fibers. The original cores of the pulled out part become vanishingly small, their stretched diameters being only about 1/100 the original diameters; the cladding of the original fibers becomes the core of the coupling section. Such a long thin coupler is very cumbersome and fragile. Furthermore, such a coupler is lossy since the original cladding takes the place of the vanished cores. In the region of the coupler where the fiber cores taper from their "vanishingly small" size to their full size, an insufficient amount of power is transferred from the cladding back to the core. Moreover, it is difficult to maintain the cores straight and parallel to one another when fibers are inserted into a tube which is then stretched unless specific steps are taken to position the fibers. Such non-linear coupler cores can lead to decreased coupling efficiency in single-mode couplers.
Japanese published application 60-140208 teaches a coupler formed by pre-twisting a pair of fibers, inserting them into a quartz tube, and heating and drawing the central part of the tube to reduce its diameter. Resin is then applied to the ends of the tube to seal the fibers thereto. This coupler has the following disadvantages. During the collapse of the tube onto the fibers, the capillary tube is not evacuated and the fibers are not held taut. The fibers therefore meander in the tube, thereby preventing the achievement of a predetermined coupling when the tube is elongated by a predetermined length. This also makes it difficult to achieve low coupler loss. The fibers are pre-twisted in order to provide a sufficient length of fiber-to-fiber contact to provide adequate coupling. Such a coupler cannot maintain the polarization of an input optical signal; moreover, it is difficult to make wavelength division multiplexed couplers with twisted fibers.